Ring formation four-heteroatomic rings

In this chapter, the theoretical methods, crystal structures, characteristic nuclear magnetic resonances, reactivities, and formation of four-membered rings with two heteroatoms including selenium or tellurium that have appeared in the last decade are described. 

The methylene or heteroatom bridges in V or VII can be replaced with methyne carbon atoms. In this case (VII), we have two systems of conjugated electrons with aromatic features. A 26-electron n = 6) aromatic system is formed by the outer double bonds, whereas the inner double bonds and M-atoms form another conjugated system, M-N-C-C-N-M. The inner system can exist in two forms. For four-valent metals, we have structure VIII and for two-valent metal we have structure VII. In chelate VII, two d-electrons of M are required for the formation of an aromatic conjugated six-electron ring. In chelate VIII, empty d-orbitals participate in the ring formation. Quantum calculations may answer the question which structure is more favorable thermodynamically. Apparently, the answer depends on the nature of the metal atoms and their axial ligands. 

Finally, we note that all of the discussed heterocyclic derivatives of indane and indene have heteroatoms only in their five-membered ring. There are many other heterocycles related to indane that have the heteroatoms located elsewhere. However, the thermochemistry of such species is essentially uncharted. The enthalpy of formation of purine (XLVIII), with its four nitrogens dispersed through both rings, has been measured in recent times [78], and chronicled in the archive [15] with yet a later value [79], These two values inexplicably differ by over 2000 kJ mol-1. In the absence of a value for the enthalpy of sublimation we are hesitant to discuss this species further, other than to note two estimates from a paper more than 100 years old [80] that straddle the results in [78] by ca. 20 kJ mol-1. 

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